Male and female B6C3F1 mice were exposed by inhalation to 0, 12.8, 32, or 80 ppm chloroprene for 104 weeks. There were increased incidences of alveolar/bronchiolar adenomas and carcinomas in lungs of female and male exposed to 12.8, 32 or 80 ppm when compared with controls. There was strong exposure-concentration-dependent response, which was especially noticeable when comparing incidences of multiple alveolar bronchiolar adenomas or carcinonas in mice in exposed and control group. K-ras mutation profiles may add some interesting information on mutation frequency and spectra and dose response relationships. This data will be presented in the NTP peer review in December 1996.